What is common descent? Why is it important to the theory of evolution?

Along path leads from the origins of primitive "life," which existed at least 3.5 billion years ago, to the profusion and diversity of life that exists today. This path is all-time understood as a product of evolution.

Contrary to pop opinion, neither the term nor the idea of biological evolution began with Charles Darwin and his foremost work, On the Origin of Species by Means of Natural Option (1859). Many scholars from the ancient Greek philosophers on had inferred that similar species were descended from a common ancestor. The word "development" first appeared in the English language language in 1647 in a nonbiological connection, and information technology became widely used in English for all sorts of progressions from simpler beginnings. The term Darwin nigh frequently used to refer to biological evolution was "descent with modification," which remains a good brief definition of the procedure today.

Darwin proposed that evolution could be explained past the differential survival of organisms following their naturally occurring variation—a process he termed "natural selection." According to this view, the offspring of organisms differ from i another and from their parents in ways that are heritable—that is, they tin pass on the differences genetically to their own offspring. Furthermore, organisms in nature typically produce more offspring than can survive and reproduce given the constraints of nutrient, space, and other ecology resource. If a particular offspring has traits that requite it an reward in a particular environment, that organism will be more than likely to survive and pass on those traits. Every bit differences accumulate over generations, populations of organisms diverge from their ancestors.

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Charles Darwin arrived at many of his insights into development by studying the variations among species on the Galápagos Islands off the coast of Ecuador.

Darwin's original hypothesis has undergone extensive modification and expansion, but the central concepts stand firm. Studies in genetics and molecular biology—fields unknown in Darwin'due south time—accept explained the occurrence of the hereditary variations that are essential to natural selection. Genetic variations consequence from changes, or mutations, in the nucleotide sequence of Deoxyribonucleic acid, the molecule that genes are made from. Such changes in DNA now can be detected and described with great precision.

Genetic mutations ascend by chance. They may or may non equip the organism with ameliorate means for surviving in its environs. But if a gene variant improves adaptation to the environment (for example, by allowing an organism to make better use of an available food, or to escape predators more finer—such as through stronger legs or disguising coloration), the organisms carrying that factor are more likely to survive and reproduce than those without it. Over fourth dimension, their descendants will tend to increase, irresolute the average characteristics of the population. Although the genetic variation on which natural selection works is based on random or chance elements, natural selection itself produces "adaptive" alter—the very reverse of take a chance.

Scientists likewise take gained an understanding of the processes by which new species originate. A new species is one in which the individuals cannot mate and produce viable descendants with individuals of a preexisting species. The split up of 1 species into ii often starts because a group of individuals becomes geographically separated from the rest. This is especially apparent in distant remote islands, such as the Galápagos and the Hawaiian archipelago, whose great distance from the Americas and Asia ways that arriving colonizers will accept lilliputian or no opportunity to mate with individuals remaining on those continents. Mountains, rivers, lakes, and other natural barriers also account for geographic separation between populations that in one case belonged to the same species.

One time isolated, geographically separated groups of individuals go genetically differentiated as a outcome of mutation and other processes, including natural selection. The origin of a species is often a gradual procedure, and so that at first the reproductive isolation between separated groups of organisms is only partial, but it somewhen becomes complete. Scientists pay special attention to these intermediate situations, because they assistance to reconstruct the details of the process and to identify particular genes or sets of genes that business relationship for the reproductive isolation between species.

A particularly compelling example of speciation involves the 13 species of finches studied by Darwin on the Galápagos Islands, now known every bit Darwin's finches. The ancestors of these finches appear to accept immigrated from the Southward American mainland to the Galápagos. Today the different species of finches on the island have singled-out habitats, diets, and behaviors, merely the mechanisms involved in speciation proceed to operate. A research group led by Peter and Rosemary Grant of Princeton University has shown that a unmarried year of drought on the islands can bulldoze evolutionary changes in the finches. Drought diminishes supplies of easily cracked nuts but permits the survival of plants that produce larger, tougher nuts. Droughts thus favor birds with stiff, broad beaks that can break these tougher seeds, producing populations of birds with these traits. The Grants have estimated that if droughts occur near once every 10 years on the islands, a new species of finch might ascend in only nigh 200 years.

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The different species of finches on the Galápagos Islands, at present known every bit Darwin'southward finches, have unlike-sized beaks that accept evolved to take advantage of distinct food sources.

The following sections consider several aspects of biological development in greater particular, looking at paleontology, comparative anatomy, biogeography, embryology, and molecular biology for further evidence supporting evolution.

The Fossil Record

Although information technology was Darwin, above all others, who commencement marshaled disarming bear witness for biological evolution, earlier scholars had recognized that organisms on Earth had inverse systematically over long periods of time. For example, in 1799 an engineer named William Smith reported that, in undisrupted layers of stone, fossils occurred in a definite sequential order, with more than modern-appearing ones closer to the pinnacle. Because lesser layers of rock logically were laid downwardly earlier and thus are older than peak layers, the sequence of fossils also could exist given a chronology from oldest to youngest. His findings were confirmed and extended in the 1830s past the paleontologist William Lonsdale, who recognized that fossil remains of organisms from lower strata were more primitive than the ones above. Today, many thousands of aboriginal rock deposits accept been identified that show corresponding successions of fossil organisms.

Thus, the full general sequence of fossils had already been recognized before Darwin conceived of descent with modification. Only the paleontologists and geologists before Darwin used the sequence of fossils in rocks not every bit proof of biological evolution, but as a basis for working out the original sequence of rock strata that had been structurally disturbed by earthquakes and other forces.

In Darwin's time, paleontology was still a rudimentary science. Big parts of the geological succession of stratified rocks were unknown or inadequately studied.

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A geological cross section of the Grand Staircase-Escalante National Monument in Utah shows layers of sedimentary rock. These layers reveal deposits laid downward over millions of years. Older fossils are constitute in the lower layers, revealing the succession (more...)

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Weathering has exposed layers of sedimentary stone near the Paria River in Utah

Darwin, therefore, worried about the rarity of intermediate forms between some major groups of organisms.

Today, many of the gaps in the paleontological record take been filled by the research of paleontologists. Hundreds of thousands of fossil organisms, found in well-dated stone sequences, represent successions of forms through time and manifest many evolutionary transitions. As mentioned earlier, microbial life of the simplest blazon was already in beingness 3.v billion years agone. The oldest evidence of more than complex organisms (that is, eucaryotic cells, which are more circuitous than bacteria) has been discovered in fossils sealed in rocks approximately 2 billion years old. Multicellular organisms, which are the familiar fungi, plants, and animals, have been institute only in younger geological strata. The following list presents the order in which increasingly complex forms of life appeared:

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Life Form	Millions of Years Since First Known Appearance (Approximate)
Microbial (procaryotic cells)	3,500
Complex (eucaryotic cells)	two,000
First multicellular animals	670
Shell-bearing animals	540
Vertebrates (simple fishes)	490
Amphibians	350
Reptiles	310
Mammals	200
Nonhuman primates	60
Earliest apes	25
Australopithecine ancestors of humans	v
Modern humans	0.15 (150,000 years)

And so many intermediate forms take been discovered betwixt fish and amphibians, between amphibians and reptiles, between reptiles and mammals, and along the primate lines of descent that it oft is hard to identify categorically when the transition occurs from one to another particular species. Actually, near all fossils can be regarded every bit intermediates in some sense; they are life forms that come betwixt the forms that preceded them and those that followed.

The fossil tape thus provides consistent evidence of systematic change through time—of descent with modification. From this huge torso of evidence, information technology can be predicted that no reversals will be found in future paleontological studies. That is, amphibians volition non announced before fishes, nor mammals before reptiles, and no complex life volition occur in the geological record before the oldest eucaryotic cells. This prediction has been upheld by the evidence that has accumulated until now: no reversals have been found.

Common Structures

Inferences about common descent derived from paleontology are reinforced by comparative anatomy. For example, the skeletons of humans, mice, and bats are strikingly like, despite the different ways of life of these animals and the diversity of environments in which they flourish. The correspondence of these animals, bone by bone, can be observed in every part of the torso, including the limbs; yet a person writes, a mouse runs, and a bat flies with structures built of bones that are different in detail but like in general structure and relation to each other.

Scientists call such structures homologies and take concluded that they are all-time explained by common descent. Comparative anatomists investigate such homologies, not but in bone construction only likewise in other parts of the body, working out relationships from degrees of similarity. Their conclusions provide important inferences about the details of evolutionary history, inferences that tin be tested past comparisons with the sequence of ancestral forms in the paleontological record.

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A bat wing, a mouse forelimb, and a human arm serve very different purposes, merely they accept the same basic components The similarities arise because all three species share a mutual 4-limbed vertebrate ancestor

The mammalian ear and jaw are instances in which paleontology and comparative anatomy combine to prove common ancestry through transitional stages. The lower jaws of mammals contain only one bone, whereas those of reptiles take several. The other bones in the reptile jaw are homologous with bones now found in the mammalian ear. Paleontologists have discovered intermediate forms of mammal-similar reptiles (Therapsida) with a double jaw joint—1 composed of the bones that persist in mammalian jaws, the other consisting of bones that eventually became the hammer and anvil of the mammalian ear.

The Distribution of Species

Biogeography besides has contributed evidence for descent from common ancestors. The diversity of life is stupendous. Approximately 250,000 species of living plants, 100,000 species of fungi, and 1 meg species of animals have been described and named, each occupying its own peculiar ecological setting or niche; and the demography is far from complete. Some species, such as human beings and our companion the dog, can live under a wide range of environments. Others are amazingly specialized. One species of a mucus (Laboulbenia) grows exclusively on the rear portion of the covering wings of a single species of beetle (Aphaenops cronei) found merely in some caves of southern France. The larvae of the fly Drosophila carcinophila tin can develop only in specialized grooves below the flaps of the third pair of oral appendages of a state crab that is establish but on certain Caribbean islands.

How can we make intelligible the colossal diverseness of living beings and the existence of such extraordinary, seemingly whimsical creatures equally the fungus, beetle, and wing described above? And why are island groups like the Galápagos so often inhabited past forms similar to those on the nearest mainland only belonging to different species? Evolutionary theory explains that biological diverseness results from the descendants of local or migrant predecessors becoming adapted to their various environments. This explanation can exist tested by examining present species and local fossils to see whether they accept similar structures, which would indicate how one is derived from the other. Also, there should be evidence that species without an established local beginnings had migrated into the locality.

Wherever such tests accept been carried out, these conditions have been confirmed. A good example is provided by the mammalian populations of North and Due south America, where strikingly different native organisms evolved in isolation until the emergence of the isthmus of Panama approximately iii 1000000 years ago. Thereafter, the armadillo, porcupine, and opossum—mammals of South American origin—migrated north, along with many other species of plants and animals, while the mountain lion and other North American species made their style across the isthmus to the southward.

The testify that Darwin found for the influence of geographical distribution on the evolution of organisms has get stronger with advancing knowledge. For example, approximately 2,000 species of flies belonging to the genus Drosophila are now found throughout the world. Near i-quarter of them live just in Hawaii.

Effigy

Until about 3 million years ago, North and South America were separated past a wide expanse of h2o, so mammals on the two continents evolved separately. After the isthmus of Panama formed, armadillos and opossums migrated north, and mountain lions migrated (more than...)

More than than a thousand species of snails and other country mollusks likewise are found just in Hawaii. The biological explanation for the multiplicity of related species in remote localities is that such slap-up diversity is a outcome of their evolution from a few mutual ancestors that colonized an isolated surroundings. The Hawaiian Islands are far from any mainland or other islands, and on the ground of geological testify they never take been attached to other lands. Thus, the few colonizers that reached the Hawaiian Islands found many available ecological niches, where they could, over numerous generations, undergo evolutionary change and diversification. No mammals other than ane bat species lived in the Hawaiian Islands when the starting time human settlers arrived; similarly, many other kinds of plants and animals were absent.

The Hawaiian Islands are not less hospitable than other parts of the globe for the absent species. For example, pigs and goats have multiplied in the wild in Hawaii, and other domestic animals also thrive in that location. The scientific explanation for the absence of many kinds of organisms, and the peachy multiplication of a few kinds, is that many sorts of organisms never reached the islands, considering of their geographic isolation. Those that did reach the islands diversified over time because of the absence of related organisms that would compete for resources.

Similarities During Evolution

Embryology, the study of biological development from the time of conception, is another source of independent evidence for common descent. Barnacles, for instance, are sedentary crustaceans with niggling credible similarity to such other crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through a free-pond larval phase in which they look like other crustacean larvae. The similarity of larval stages supports the conclusion that all crustaceans take homologous parts and a common ancestry.

Similarly, a wide variety of organisms from fruit flies to worms to mice to humans take very like sequences of genes that are active early in evolution. These genes influence trunk segmentation or orientation in all these diverse groups. The presence of such similar genes doing similar things across such a wide range of organisms is best explained by their having been present in a very early common ancestor of all of these groups.

New Show from Molecular Biology

The unifying principle of common descent that emerges from all the foregoing lines of show is being reinforced by the discoveries of modernistic biochemistry and molecular biology.

The lawmaking used to translate nucleotide sequences into amino acid sequences is essentially the same in all organisms. Moreover, proteins in all organisms are invariably composed of the aforementioned set of 20 amino acids. This unity of composition and function is a powerful argument in favor of the common descent of the almost diverse organisms.

In 1959, scientists at Cambridge University in the United Kingdom determined the three-dimensional structures of two proteins that are found in almost every multicelled animal: hemoglobin and myoglobin. Hemoglobin is the protein that carries oxygen in the blood. Myoglobin receives oxygen from hemoglobin and stores information technology in the tissues until needed. These were the first three-dimensional protein structures to be solved, and they yielded some cardinal insights. Myoglobin has a single chain of 153 amino acids wrapped effectually a group of fe and other atoms (chosen "heme") to which oxygen binds. Hemoglobin, in contrast, is made of up iv chains: two identical chains consisting of 141 amino acids, and 2 other identical bondage consisting of 146 amino acids. However, each chain has a heme exactly like that of myoglobin, and each of the 4 chains in the hemoglobin molecule is folded exactly like myoglobin. It was immediately obvious in 1959 that the 2 molecules are very closely related.

During the side by side two decades, myoglobin and hemoglobin sequences were adamant for dozens of mammals, birds, reptiles, amphibians, fish, worms, and molluscs. All of these sequences were then obviously related that they could be compared with confidence with the 3-dimensional structures of two selected standards—whale myoglobin and horse hemoglobin. Even more significantly, the differences betwixt sequences from different organisms could be used to construct a family tree of hemoglobin and myoglobin variation amidst organisms. This tree agreed completely with observations derived from paleontology and anatomy about the mutual descent of the corresponding organisms.

Effigy

Myoglobin, which stores oxygen in muscles, consists of a chain of 153 amino acids wrapped effectually an oxygen-binding molecule. The sequence of amino acids in myoglobin vanes from species to species, revealing the evolutionary relationships amongst organisms. (more than...)

Similar family histories have been obtained from the three-dimensional structures and amino acid sequences of other proteins, such equally cytochrome c (a protein engaged in energy transfer) and the digestive proteins trypsin and chymotrypsin. The test of molecular construction offers a new and extremely powerful tool for studying evolutionary relationships. The quantity of information is potentially huge—equally large as the thousands of dissimilar proteins contained in living organisms, and limited only past the time and resources of molecular biologists.

As the power to sequence the nucleotides making upwardly Dna has improved, it also has become possible to utilise genes to reconstruct the evolutionary history of organisms. Because of mutations, the sequence of nucleotides in a gene gradually changes over time. The more closely related two organisms are, the less unlike their Deoxyribonucleic acid will exist. Because there are tens of thousands of genes in humans and other organisms, Deoxyribonucleic acid contains a tremendous corporeality of information well-nigh the evolutionary history of each organism.

Genes evolve at dissimilar rates because, although mutation is a random outcome, some proteins are much more than tolerant of changes in their amino acid sequence than are other proteins. For this reason, the genes that encode these more tolerant, less constrained proteins evolve faster The average rate at which a particular kind of gene or protein evolves gives rise to the concept of a "molecular clock." Molecular clocks run speedily for less constrained proteins and slowly for more constrained proteins, though they all time the same evolutionary events.

The figure on this page compares three molecular clocks: for cytochrome c proteins, which collaborate intimately with other macromolecules and are quite constrained in their amino acid sequences; for the less rigidly constrained hemoglobins, which interact mainly with oxygen and other minor molecules; and for fibrinopeptides, which are poly peptide fragments that are cut from larger proteins (fibrinogens) when blood clots. The clock for fibrinopeptides runs chop-chop; 1 per centum of the amino acids change in a little longer than i one thousand thousand years. At the other extreme, the molecular clock runs slowly for cytochrome c; a i percent change in amino acid sequence requires twenty 1000000 years. The hemoglobin clock is intermediate.

The concept of a molecular clock is useful for two purposes. Information technology determines evolutionary relationships amid organisms, and it indicates the time in the past when species started to diverge from i some other. In one case the clock for a particular gene or poly peptide has been calibrated by reference to some effect whose time is known, the actual chronological time when all other events occurred can be determined by examining the protein or gene tree.

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Species that diverged longer ago have more differences in their respective proteins, reflecting changes in the amino acids over fourth dimension. Proteins evolve at different rates depending on the constraints imposed by their functions. Cytochrome c, a protein (more...)

An interesting additional line of testify supporting development involves sequences of DNA known equally "pseudogenes." Pseudogenes are remnants of genes that no longer function only continue to be carried along in Dna as excess baggage. Pseudogenes too modify through time, every bit they are passed on from ancestors to descendants, and they offer an particularly useful way of reconstructing evolutionary relationships.

With functioning genes, one possible explanation for the relative similarity between genes from different organisms is that their ways of life are like—for example, the genes from a horse and a zebra could be more than similar because of their similar habitats and behaviors than the genes from a horse and a tiger. But this possible caption does non work for pseudogenes, since they perform no role. Rather, the degree of similarity betwixt pseudogenes must simply reflect their evolutionary relatedness. The more remote the last common ancestor of two organisms, the more dissimilar their pseudogenes will be.

The evidence for evolution from molecular biology is overwhelming and is growing quickly. In some cases, this molecular testify makes information technology possible to get beyond the paleontological evidence. For example, it has long been postulated that whales descended from country mammals that had returned to the ocean. From anatomical and paleontological evidence, the whales' closest living land relatives seemed to be the even-toed hoofed mammals (modem cattle, sheep, camels, goats, etc.).

Recent comparisons of some milk protein genes (beta-casein and kappa-casein) have confirmed this relationship and take suggested that the closest state-spring living relative of whales may be the hippopotamus. In this example, molecular biology has augmented the fossil record.

Creationism and the Evidence for Evolution

Some creationists cite what they say is an incomplete fossil record every bit testify for the failure of evolutionary theory. The fossil tape was incomplete in Darwin's fourth dimension, but many of the of import gaps that existed then have been filled by subsequent paleontological research. Possibly the virtually persuasive fossil testify for evolution is the consistency of the sequence of fossils from early to recent. Nowhere on Earth do we observe, for example, mammals in Devonian (the age of fishes) strata, or human being fossils coexisting with dinosaur remains. Undisturbed strata with simple unicellular organisms predate those with multicellular organisms, and invertebrates precede vertebrates; nowhere has this sequence been found inverted. Fossils from adjacent strata are more similar than fossils from temporally distant strata. The almost reasonable scientific decision that can be drawn from the fossil record is that descent with modification has taken identify as stated in evolutionary theory.

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Mammakian land antecedent Ambulocetus

Special creationists argue that "no 1 has seen development occur." This misses the betoken about how science tests hypotheses. Nosotros don't run into Globe going around the lord's day or the atoms that make upward matter. Nosotros "see" their consequences. Scientists infer that atoms be and Earth revolves because they accept tested predictions derived from these concepts past all-encompassing observation and experimentation.

Furthermore, on a minor calibration, nosotros "experience" evolution occurring every twenty-four hours. The annual changes in influenza viruses and the emergence of antibiotic-resistant bacteria are both products of evolutionary forces. Indeed, the rapidity with which organisms with short generation times, such as leaner and viruses, can evolve under the influence of their environments is of neat medical significance. Many laboratory experiments take shown that, because of mutation and natural choice, such microorganisms can modify in specific ways from those of immediately preceding generations.

On a larger scale, the evolution of mosquitoes resistant to insecticides is another example of the tenacity and adaptability of organisms nether environmental stress. Similarly, malaria parasites have become resistant to the drugs that were used extensively to combat them for many years. Every bit a consequence, malaria is on the increase, with more than than 300 meg clinical cases of malaria occurring every twelvemonth.

Molecular evolutionary information counter a recent proposition called "intelligent design theory." Proponents of this idea debate that structural complexity is proof of the direct hand of God in particularly creating organisms as they are today. These arguments repeat those of the 18th century cleric William Paley who held that the vertebrate eye, considering of its intricate organization, had been specially designed in its nowadays grade by an omnipotent Creator. Modem-twenty-four hour period intelligent design proponents argue that molecular structures such as DNA, or molecular processes such every bit the many steps that claret goes through when it clots, are so irreducibly complex that they tin can function only if all the components are operative at in one case. Thus, proponents of intelligent pattern say that these structures and processes could not have evolved in the stepwise mode characteristic of natural selection.

Effigy

Rodhocetus Balaenoptera (modern Blue whale)

All the same, structures and processes that are claimed to be "irreducibly" complex typically are not on closer inspection. For example, it is incorrect to assume that a circuitous structure or biochemical process tin can function only if all its components are present and functioning as we see them today. Complex biochemical systems tin exist built upwardly from simpler systems through natural choice. Thus, the "history" of a protein can be traced through simpler organisms. Jawless fish have a simpler hemoglobin than practice jawed fish, which in plow accept a simpler hemoglobin than mammals.

The evolution of complex molecular systems tin can occur in several ways. Natural choice can bring together parts of a system for one part at ane time and and then, at a later time, recombine those parts with other systems of components to produce a system that has a different function. Genes can be duplicated, altered, and then amplified through natural selection. The complex biochemical cascade resulting in blood clotting has been explained in this fashion.

Similarly, evolutionary mechanisms are capable of explaining the origin of highly complex anatomical structures. For example, eyes may accept evolved independently many times during the history of life on Earth. The steps keep from a simple heart spot made up of light-sensitive retinula cells (as is now found in the flatworm), to germination of individual photosensitive units (ommatidia) in insects with light focusing lenses, to the eventual germination of an eye with a single lens focusing images onto a retina. In humans and other vertebrates, the retina consists not only of photoreceptor cells but also of several types of neurons that begin to analyze the visual image. Through such gradual steps, very unlike kinds of optics take evolved, from unproblematic lite-sensing organs to highly complex systems for vision.

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Optics evolved over many millions of years from simple organs that tin notice light.

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Source: https://www.ncbi.nlm.nih.gov/books/NBK230201/

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